It is known that aluminum hydroxide can be used as a resin filler. Especially, it is well-known that aluminum hydroxide is filled in an unsaturated polyester resin or an acrylic resin and an onyx-like or marble-like molded article is prepared from the resin composition. Aluminum hydroxide is expressed by the chemical formula of Al(OH).sub.3 or Al.sub.2 O.sub.3.3H.sub.2 O, and at a temperature higher than 200.degree. C., water vapor is released from the interior of the crystal and a large endothermism is shown at this point. Accordingly, if aluminum hydroxide is used as a resin filler, an excellent flame retardancy can be obtained. Moreover, aluminum hydroxide has an excellent reduced fuming property, an excellent arc resistance, and an excellent tracking resistance, and the manufacturing cost is low. Accordingly, aluminum hydroxide can be regarded as a very valuable flame retardant.
Recently, to improve the surface smoothness, the boiling resistance, the flame retardancy, and the sedimentation resistance in the best state, aluminum hydroxide having a fine particle size has been used.
When obtaining a molded article by press-molding BMC (bulk molding compound) or SMC (sheet molding compound), the use of aluminum hydroxide having a fine particle size advantageously prevents the separation of the filler from the resin at the pressing step. Accordingly, aluminum hydroxide having a particle size reduced by pulverization is widely used as a resin filler. In pulverized aluminum hydroxide, however, the crystals is destroyed and large quantities of fine particles formed by chipping are contained, and the specific surface area of the powder is large, and accordingly, a problem of a large moisture adsorption arises. In some applications, the use of aluminum hydroxide having a large moisture adsorption as a filler results in problems such as an insufficient dispersion of the filler, insufficient curing of the resin, reduction of the hardness of the molded article, insufficient insulating properties and foaming at the kneading step, and thus the use of this aluminum hydroxide is not preferable in some cases. Generally, with respect to a pulverization product, the finer the filler, the larger the oil absorption of the filler, and the filling of the filler into the resin at a high ratio becomes difficult.
Furthermore, an unsaturated polyester resin comprising aluminum hydroxide having a large specific surface area as a filler involves problems in that an oil-soluble curing promoter such as cobalt naphthenate is trapped on the surface of aluminum hydroxide, with the result that the gel time is delayed, the productivity is reduced, and the molded article is yellowed.
These problems are solved to some extent by surface-treating pulverized aluminum hydroxide with a surface-treating agent such as stearic acid, a metal salt thereof or a silane coupling agent, but this surface treatment results in increased costs.
A process has long been known in which aluminum hydroxide having a fine particle size is precipitated by using an alumina gel disclosed in U.S. Pat. No. 2,549,549 or French Patent No. 2,041,750 as a precipitation-inducing material. Fine aluminum hydroxide prepared according to this process has a smaller specific surface area than that of pulverized aluminum hydroxide having a similar particle size and the water adsorption is smaller. This aluminum hydroxide, however, is in the form of secondary particles formed by an agglomeration of fine primary particles and has a very large oil absorption. Accordingly, it is very difficult to fill aluminum hydroxide as a filler in an amount sufficient to impart a required flame retardancy to a resin. Japanese Unexamined Patent Publication No. 59-501711 discloses a process for precipitating aluminum hydroxide having a fine particle size by using pulverized aluminum hydroxide as a precipitation-inducing material. It has been also found that aluminum hydroxide obtained according to this process has a very large oil absorption.